To remove failed host (which is no longer exist, bacause is the totally crashed) try this on the pool master:
Determine if there is no VM running state on the filed host:
xe vm-list
If it is running reset the VM power state:
xe vm-reset-powerstate force=true vm=<uuid-of-the-running-vm-on-the-failed-host>
Destroy, forget failed host:
xe host-list xe host-forget uuid=<uuid-of-the-failed-slave>
If there will still remain local storages of the removed host in the pool, forget them:
xe sr-forget uuid=<UUID>